I've been doing prenatal yoga classes at least once a week since week 10. Best decision ever. I take classes at Puma Yoga in Lakewood. The class is all pregnant women so we concentrate a lot on hips, back and shoulders, as well as building leg and arm strength. I highly recommend for any pregnant woman to take a prenatal yoga class. We also do a lot of breathing techniques that are supposed to help with labor. And, you can obviously do the moves on your own time anytime you need to stretch out your back or hips.
I also got a prenatal yoga video on amazon that is pretty good. I think you'll get more out of a class but if you want to try a video at home first, this one is only $10.